Generally, when using a demodulator demodulating a modulated signal, for example, a frequency modulated signal (hereinafter referred to as an FM signal), in a wireless communication device, an FM demodulation circuit and a filter circuit or a phase shifter are required. In addition, along with miniaturization of a device having a demodulator in recent years, such as a wireless communication device, the downsizing of the demodulator has been promoted.
As a means to downsize the demodulator, it can be considered to integrate an FM demodulation circuit and a filter circuit. However, when the FM demodulation circuit and the filter circuit are integrated in the demodulator, absolute accuracies of a resistance value of a register, a capacity value of a condenser, etc. in this integrated circuit (hereinafter referred to as an IC) are not satisfactory, causing a deviation in demodulation sensitivity and frequency characteristic for each IC.
This is because a frequency characteristic of the filter circuit or the phase shifter used in the demodulator demodulating the FM signal generally depends on a product CR, that is, the product of a capacity value (C) of a condenser and a resistance value (R) of a resistor which constitute the filter circuit or the phase shifter.
Conventionally, in order to eliminate such dispersions in the characteristics of the respective ICs, a circuit has been proposed in which the demodulation sensitivity of an FM demodulation circuit and the frequency characteristic of a filter circuit or a phase shifter in a demodulator are automatically adjusted by utilizing that relative accuracies of a resistance value of a resistor and a capacity value of a condenser are satisfactory compared with absolute accuracies of the same in an identical IC.
As a circuit for automatically adjusting the demodulation sensitivity and the frequency characteristic, for example, “an FM signal demodulation circuit” is disclosed in Japanese Unexamined Patent Publication No. 7-115328/1995 (Tokukaihei 7-115328, published on May 2, 1995). FIG. 14 schematically shows a demodulator having the FM signal demodulation circuit.
As shown in FIG. 14, the demodulator is structured so as to include an input terminal 101 to which an FM signal is inputted; an FM demodulation circuit 104 for demodulating the FM signal inputted to the input terminal 101, constituted by a phase shifter 108 and a phase comparator 109; an output terminal 106 for outputting the signal demodulated by the FM demodulation circuit 104; and a CTRL circuit 105 for controlling a characteristic of the phase shifter 108 included in the FM demodulation circuit 104 and outputting a control signal for stabilizing demodulation sensitivity.
Here, the CTRL circuit 105 is a circuit for generating the control signal for stabilizing the demodulation sensitivity, and is arranged so as to automatically adjust the demodulation sensitivity and the frequency characteristic in the FM demodulation circuit 104 by the generated control signal.
Meanwhile, the CTRL circuit 105 of the demodulator shown in FIG. 14 is a large-scale circuit, as it includes a reference circuit having a circuit structure in the same way as the phase shifter 108 and the FM demodulation circuit 104. This arrangement causes increases in circuit scale and current consumption of the demodulator as a whole. Besides, although a relative accuracy is satisfactory compared with an absolute accuracy in an identical IC, a dispersion occurs in the demodulation sensitivity of the FM demodulation circuit, due to relative errors in the resistance value of the resistor and the capacity value of the condenser inside the IC.